I'm looking to install a set of Photon Blasters on my front forks and will need some clamps for them. Does anybody know the dimension of the lower (outer) fork tube?

And that's at the lower end (larger diameter) of the forks where the front wheel attaches, not the upper end (smaller diameter) where the turn signals attach, correct?

I have just been outside and measured it out.
It is about 50mm. At the lowest point I measured 50,3mm and higher, just below where it gets thicker (slide and gasket part) it was about 49,5.

So a 50mm clamp will do it.